The use of crosslinkable polymeric compositions to fabricate articles such as cables, pipes, footwear, and foams is generally well known. Crosslinkable polymeric composition generally include a polymer, a crosslinking agent such as organic peroxides, antioxidants, and optionally, various other additives such as a scorch inhibitor and a crosslinking booster. Crosslinking assists the polymer in meeting mechanical and physical requirements such as improved high temperature properties.
The crosslinking of polymers with free radical initiators such as organic peroxides is also generally well known. Generally, the organic peroxide is incorporated into the polymer by melt blending in a roll mill, a biaxial screw kneading extruder, or a Banbury™ or Barbender™ mixer at a temperature lower than the onset temperature for significant decomposition of the peroxide. Peroxides are judged for decomposition based on their half-life temperatures as described in Plastic Additives Handbook, Gachter et al, 1985, pages 646 to 649.
An alternative method for organic peroxide incorporation into a polymeric compound is to mix liquid peroxide and pellets of the polymer in a blending device, such as a Henschel™ mixer or a soaking device such as a simple drum tumbler, which are maintained at temperatures above the freeze point of the organic peroxide and below the decomposition temperature of the organic peroxide and the melt temperature of the polymer. Following the organic peroxide incorporation, the polymer/organic peroxide blend is then, for example, introduced into an extruder where it is extruded around an electrical conductor at a temperature lower than the decomposition temperature of the organic peroxide to form a cable. The cable is then exposed to higher temperatures at which the organic peroxide decomposes to provide free radicals, which crosslink the polymer.
Polymers containing peroxides are vulnerable to scorch (premature crosslinking occurring during the extrusion process). Scorch causes the formation of discolored gel-like particles in the resin and leads to undesired build up of extruder pressure during extrusion. Further, to achieve a high crosslink density, high levels of organic peroxide have often been used. However, high levels of organic peroxide leads to a problem known as sweat-out, which has a negative effect on the extrusion process and the final product. Sweat-out dust is a potential explosion hazard, may foul filters, and can cause slippage and instability in the extrusion process. Therefore, it is desired to reduce the amount of organic peroxide while maintaining a high crosslink density.
U.S. Pat. No. 6,143,822 discloses a polyethylene crosslinkable composition comprising (a) polyethylene; (b) 1,1-diphenylethylene, substituted or unsubstituted, as a scorch inhibitor; (c) optionally, a cure booster; and (d) an organic peroxide.
U.S. Pat. No. 6,187,847 discloses a polyethylene crosslinkable composition comprising (a) polyethylene; (b) [1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione], as a scorch inhibitor; and (c) an organic peroxide.
U.S. Pat. No. 6,228,917 discloses a polyethylene crosslinkable composition comprising (a) polyethylene; (b) 4,4′-thiobis(2-methyl-6-t-butylphenol); 2,2′-thiobis(6-t-butyl-4-methyphenol); or mixture thereof, as the scorch inhibitor; (c) optionally, a cure booster; and (d) an organic peroxide.
U.S. Pat. No. 6,656,986 discloses a polyethylene crosslinkable composition comprising (a) polyethylene; (b) [1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione], as a scorch inhibitor; (c) a thioester; (d) a hindered amine stabilizer; and (e) an organic peroxide.
Despite the research efforts in developing crosslinkable polyethylene compositions, there is still a need for improved crosslinkable polyethylene compositions, which can be extruded at high temperatures and high rate with minimum scorch and yet be crosslinked at a fast cure rate to a high crosslink density without sacrificing long-term heat aging stability. Furthermore, there is still a need for a method of making such improved crosslinkable polyethylene compositions.